Author: Dieter Buerssner
Date: 17:10:38 10/26/02
Go up one level in this thread
On October 25, 2002 at 21:27:18, Nagendra Singh Tomar wrote:
>score = -alphabeta(board, -alpha-1, -alpha, depth-1);
>
>if(score > alpha && score < beta)
>{
> score = -alphabeta(board, -beta, -score, depth-1);
> OR
> score = -alphabeta(board, -beta, -score+1, depth-1);
>}
>
>since the zero window search failed high and it returns
> score, this means that the true score is >= score.
> Now when we do a wide window search, if we want to store the PV
>properly we have to pass alpha value less than the possible
> score, else PV will not get updated (Note: PV gets updated only
> for nodes where score > alpha.). So we pass -score+1
>
>Am I right ?
Yes.
Using -score instead of -score+1 would also be correct in another sense. For a
"pure" alpha-beta algorithm (including PVS) it would calculate the correct
score, allways. But, as you noted, you will miss the PV now and then. IIRC the
original paper of Reinfeld about the Negascout algorithm used -score as bound.
With typically used enhancements of alpha-beta (extensions, pruning, HTs),
unfortunately things get a bit more complicated - as others have mentioned. To
me, it is not clear what is best to do, when you get a score > alpha and < beta
in the first call (the zero window search), and receive a score <= result of the
first call in the research (something that will never happen with a "pure"
alpha-beta search).
Jose Carlos thinks, it is better to trust the research (actually I am currently
doing this). But why should the research be more trustful than the original
search?
Regards,
Dieter
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